The present invention relates to a selective chemical sensitive FET transducer for use in detection and measurement of chemical properties, and a method for the manufacture of such selective chemical sensitive FET transducer. ("Chemical properties", as used herein, shall be understood to include ion activity and concentration, presence and concentration of enzymes, substrates, antibodies, antigens, hormones and reducible gases and the presence, concentration and activity of any of a variety of chemical and biochemical substances.)
As an instrument selectively sensitive to a chemical substance, such as hydrogen gas, sodium ions, potassium ions, calcium ions, oxygen gas and/or carbon dioxide ions, for detecting and measuring chemical properties of such substance, a positive ion sensitive glass electrode has heretofore been known. This glass electrode is, when used in the measurement of the ion activity such as often considered of prime interest in the medical or physiological field, inserted in a tissue of a living body. Because of the nature of the manner in which the glass electrode is used, various attempts have heretofore been made to make it miniature. However, the manufacture of the glass electrode of a compact or miniature size involves the following shortcomings.
(a) Since the resistance of a glass film forming the glass electrode is about 10 M, the use is required of an amplifier of a type having a high input resistance.
(b) Since the glass film is very thin, it lacks sufficient physical strength.
(c) Since the area of the glass membrane of a miniature glass electrode is small, the resistance of the glass electrode film correspondingly increases.
In view of the above, the required measurement instrument is bulky and complicated. Furthermore, not only is the glass electrode itself fragile and, therefore, susceptible to damage, but also various problems in practice have been involved when it comes to the use of the glass electrode in the tissue of the living body for detecting and measuring any substance present in such tissue of the living body.
In order to overcome the above described shortcomings of the conventional glass electrode, a recently developed solid-state measuring device is disclosed in "Development, Operation, and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology" by Piet Bergveld, IEEE Transactions of Biomedical Engineering, September, 1972, (Vol. BME 19, No. 5), pages 342-351. The ion-sensitive field-effect transistor suggested by Piet Bergveld is a modified version of the conventional metal oxide semiconductor field-effect transistor (MOSFET) wherein a metallic plate forming the gate in MOSFET has been replaced by an electrically insulating layer of a silicon dioxide which is, when the device is in use for the measurement of ion activities in a solution, held in contact with the solution, to detect a potential difference between the solution and the insulated layer. The principle of the modified MOSFET suggested by Piet Bergveld is based on the fact that the conductivity of an electroconductive channel between the source and drain regions depends on the potential difference between the insulated-gate layer, that is, silicon dioxide insulation layer, and the solution. Because of the principle of the modified MOSFET, Piet Bergveld has made it possible not only to manufacture the measuring device in a miniature size without increasing the input impedance, but also to manufacture a solid-state measuring device of a type having a plurality of such transducers for enabling measurement of different chemical properties. In particular, the solid-state measuring device suggested by Piet Bergveld is, because of the smallness of its low output being impedance, generally considered an attractive and convenient instrument for a monitoring sensor which is used as inserted in the tissue of a living body during the physiological measurement. However, since the modified MOSFET has its insulated gate of the silicon dioxide, immersion of the device in the solution results in hydration of the silicon oxide insulation layer, therefore, the accuracy of the measurement of the ion activity tends to be adversely affected once such hydration takes place.
An improved version of the modified MOSFET has fairly recently been disclosed in "An Integrated Field-Effect Electrode for Bipotential Recording" by K. D. Wise et al., IEEE Transactions on Biomedical Engineering, November, 1974, (Vol. BME 21, No. 6), pages 485-487. The device suggested by K. D. Wise et al. is similar to the above-described one, but for having a silicon nitride layer additionally formed on the insulated gate layer of silicon oxide. This device is successful in substantially eliminating the possibility of the hydration of the silicon dioxide layer, thereby enabling the accurate and effective ion activity measurement. This improved version is referred to as a liquid-oxide semiconductor field-effect transistor (LOSFET) and is useable as a sensor sensitive to hydrogen ions. In fact, K. D. Wise et al. have suggested in the same literature that the LOSFET can be used as a sensor selectivity sensitive to any one of a number of chemical substances and/or measuring various chemical properties.
The U.S. Pat. No. 4,020,830, patented on May 3, 1977, discloses a selective chemical sensitive FET transducer capable of selectively detecting and measuring chemical properties of substances to which the transducer is exposed. This chemical sensitive FET transducer has an insulated gate layer, such as silicon dioxide or silicon nitride, on which a chemical selective system adapted to interact selectively with certain ions, such as sodium ion, potassium ion and so on when exposed to a solution containing those ions, as in the case of the glass electrode, is overlaid.
However, in order for any one of the above described conventional FET transducers to be operable in a steady manner even when immersed in an aqueous solution to which it is exposed, the surfaces of the silicon substrate are defined by trimming the silicon wafer along the scribed lines during the manufacture of silicon nitride (Si.sub.3 N.sub.4), and covered with a layer of epoxy resin. While the epoxy resin layer must have a relatively small thickness in order for the transducer to be assembled in a miniature size, the employment of the thin epoxy resin layer is susceptible to dielectric break down and, therefore, the transducer cannot be used in practice for a prolonged period of time. Moreover, the insulation by the use of the epoxy resin layer is for the purpose of experiments and cannot be employed in the manufacture of the transducer on a mass-production basis.
In an attempt to make it possible to manufacture a chemical sensitive FET transducer on a mass-production basis, two of the inventors of the present invention have successfully developed an attractive and convenient method for the manufacture of the chemical sensitive FET transducer which is disclosed in the Japanese Laid-open Patent Publication No. 25385/78, laid open to public inspection on Mar. 9, 1978. The manufacturing method disclosed in the Japanese Laid-open Patent Publication is characterized in the employment of a photoetching technique to enable the FET transducer to be fabricated by the use of any known integrated circuit board manufacturing process in combination with the silicon etching process. More specifically, according to the method disclosed in the Japanese Laid-open Patent Publication, the photoetching technique is applied to form diffusion layer of drain, source, channel stopper and so on, on one surface of a silicon wafer and a layer of silicon dioxide is subsequently deposited on the FET transducers followed by the formation of a layer of Si.sub.3 N.sub.4.
Thereafter, respective portions of the Si.sub.3 N.sub.4 and SiO.sub.2 layers and silicon wafer are etched sequentially in the order given above to form, for example, a comb-shaped cut-out portion in the silicon wafer and, subsequently, the resultant exposed surfaces of the silicon wafer, which define the cut-out portion referred to above, are coated with an insulation layer of silicon dioxide (SiO.sub.2). Finally, a chemical selective membrane adapted to selectively interact with any particular ion is overlaid on the gate region, thereby providing complete selective chemical sensitive FET transducers. Since the silicon etching technique is employed in this method, the manufacture of the transducers can be effectively achieved in a miniature size and on a mass-production basis, and also insulation of the fine tip of each transducer can easily be achieved. Moreover, since this method does not require the coating of exposed surfaces of the silicon substrate adjacent the gate regions with an electrically insulating material such as epoxy resin, it is therefore especially suitable to manufacture miniature FET transducers.
However, it has been found that the FET transducer manufactured by the method of the Japanese Laid-open Patent Publication involves the following shortcomings.
(A) Since the method is such that, after the field-effect transistors have been fabricated on the silicon wafer, a portion of the silicon wafer adjacent to gate regions is cut out by the use of a chemical etching technique, there is a relatively great possibility that the gate defining insulation layer is likely to be damaged or deteriorated during said chemical etching process of the silicon wafer.
(B) Possibly because it is difficult to achieve a complete insulation by the oxide layer coated over such exposed surfaces of the silicon wafer as formed by trimming to provide separate transducers, the insulating property of the oxide layer tends to be deteriorated and/or an electric signal drift is susceptible to occur during the continued use of the FET transducer for a prolonged period of time.
(C) The FET transducer so manufactured tends to be affected by the temperature change and the extent to which they are affected by the temperature change varies from one transducer to another. For example, the FET transducer so manufactured is used in the medical and physiological field, in particular, for detecting and/or measuring chemical components such as hydrogen, sodium and potassium ions in the tissue of the human body as well as blood etc. The detection of pH value down to one place of decimals (corresponding about 6 mV) is generally considered very important in the case of metabolic or respiratory acidosis or alkalosis, and the FET transducer so manufactured results in considerable amount of measurement error since they usually have a temperature coefficient within the range of 0.7 to 4.5 mV/.degree.C. and, in most cases, within the range of 2 to 4 mV/.degree.C. Therefore, the FET transducer involves a problem to be solved before it is applied in practice.
(D) Reproducibility of temperature coefficients of the transducers varies from one transducer to another and, therefore, their temperature compensation can hardly be achieved in practice.